1. Field of the Invention
The present invention relates to an amplifier having distortion compensation, and more particularly to an amplifier, which is employed as a power amplifier for amplifying a linear modulated signal or a low-noise amplifier for receiving and amplifying a linear modulated signal. Further, the present invention relates to a structure of a base station for radio communication using the amplifier, which performs distortion compensation.
2. Description of the Related Art
Recently, there is a demand for employing an amplifier having high linear characteristics as a power amplifier to amplify linear modulated signals or as a low-noise amplifier employed in a receiver, which receives linear modulated signals, in order to suppress deterioration in transmission characteristics caused by spectrum characteristics or distortion of transmitted signals.
In many applications, amplifiers are also required to have high power efficiency. However, it is generally understood that linearity and efficiency of an amplifier be contradictory to each other. To satisfy both characteristics, various kinds of distortion compensating methods have been proposed.
A pre-distorter is well-known as one of the distortion compensating methods. A principle of the pre-distorter is to obtain a desired signal, of which distortion is removed, from an output of an amplifier by previously adding a characteristic reversed to the distortion characteristic of the amplifier to an input signal of the amplifier.
FIG. 1 shows a structural block diagram of a conventional pre-distorter. In FIG. 1, reference numeral 1 is a power amplifier, which amplifies an output signal from an orthogonal modulator 2. Compensation values .DELTA.I.sub.0 and .DELTA.Q.sub.0 sent from a distortion compensating table 8 are added to input base band signals I and Q in adders 6, respectively, and inputted to the orthogonal modulator 2. Orthogonal modulated signals are amplified in the power amplifier 1, and are added with distortion caused by a characteristic of the amplifier. However, the distortion can be removed by the compensating values .DELTA.I.sub.0 and .DELTA.Q.sub.0.
A part of the output signal of the amplifier 1 is modulated in an orthogonal demodulator 3 to obtain the base band signals I' and Q'. Then, a phase shifter 5 adjusts a phase of a local oscillating signal for demodulation sent from a local oscillator 4 so as to coincide phases of the base band signals I' and Q' with those of the input signals I and Q.
Comparators 7 compare the obtained signals I' and Q' with the input base band signals I and Q, and obtain .DELTA.I.sub.1 and .DELTA.Q.sub.1 for updating the values of the distortion compensating table 8 according to differences between those signals, and .DELTA.I.sub.1 and .DELTA.Q.sub.1 are then stored in the distortion compensating table 8.
In the above-described conventional method, the distortion of the amplifier 1 is obtained as an error of each component in an orthogonal coordinate group. Therefore, pre-distortion is performed by adding the characteristic reversed to that of the obtained distortion to each axis component.
However, there are some problems in the conventional method as follows;
(a) It is required for the distortion compensating table 8 to have compensating values for every points on the two-dimensional plane, expressed by the input signals I and Q, since there are included two distortions of amplitude and phase in the distortion of the amplifier 1. Therefore, 2.sup.2n .times.n bits are required as storage capacity of the distortion compensating table 8, in the case where the quantized bit number of the input signals I and Q is n bits. PA1 (b) Meanwhile, in a method for performing distortion compensation in a polar coordinate group, the compensating table 8 is formed in one-dimensional plane. However, when interfacing to the orthogonal modulator 2 and the orthogonal demodulator 3, conversion arithmetics for converting from a polar coordinate group to an orthogonal coordinate group, or an orthogonal coordinate group to a polar coordinate group is required. PA1 (c) The base band input signals I and Q should have the same phases as the demodulated signals I' and Q'. Therefore, it is required to provide a phase shifter 5 for adjusting local phases of an orthogonal detector, which is an orthogonal demodulator 3. Further, a quantity of phase rotation of the amplifier 1 varies according to operation temperature and operation period. Then, a function for keeping its condition the best is required to adaptively make the local phase respond to the operation. PA1 (d) Further, generation of delay in an amplifier or a filter causes time lag between the input signals I and Q and the demodulated signals I' and Q'. Therefore, the error signal, which is added from the distortion compensating table 8 to an input signal is not optimum for the input signal, and then, the characteristic of a linearizer deteriorates.
Meanwhile, in radio communication devices used for mobile communication, a plurality of carrier signals are commonly power-amplified in a radio frequency band, in order to perform signal transmission and the transmission of multiplied signals. Therefore, a power amplifier having good linearity is desired as the above-described power amplifier, in order to suppress deterioration of transmission characteristics.
The power amplifier having better linearity, for example, a feed forward type amplifier, has weak points, such as large scale, large consumptive power, and high cost of the device. However, there is a possibility of improving a compact device having high power efficiency with low-cost, if the distortion compensation of the power amplifier can be realized by digital signal processing.
In this case, A/D and D/A converters are indispensable. However, it is difficult to have A/D and D/A convertors, which have sufficient operation speed, and a desired accuracy (number of bits). Currently, maximum output and input frequencies are several 10 MHz, and it is necessary to employ a convertor, if a communication device is used in a frequency band at much exceeding 10 MHz.